The present invention relates to the optical fiber telecommunication networks, and more particularly, to the methods and systems for multichannel optical fiber communication.
Conventional methods for multiplexing several communication channels over the same fiber are Time Division Multiplexing (TDM) and Wavelength Division Multiplexing (WDM). In the TDM method, the number of communication channels is determined by a transmission bit rate. Bit rates of 622 Mbit/sec, 2.5 Gbit/sec and, lately, 10 Gbit/sec are conventional for modern long distance telecommunication networks.
In the WDM method, a separate transmission wavelength is assigned to each communication channel within the transparency band of the optical fiber (1500 nm or 1310 nm). All WDM channels are transmitted over the same fiber. For example, the number of WDM channels in the existing networks may exceed 40 in the optical band of 1530-1560 nm of Er-doped fiber optical amplifier, with spectral distance between the channels 0.8 nm or less.
Both TDM and WDM methods have a theoretical capacity limit related to the optical transparency band width divided by bandwidth required for each individual channel. Optical fiber has an active transparency band of 5.multidot.10.sup.13 sec.sup.-1 in the proximity of 1550 nm. In this transparency band, over 1000 channels having 2.5 Gbit/sec rate may be transmitted.
However, practically available TDM and WDM systems have capacities substantially below the theoretical limit. The maximum TDM bit rate is limited by maximum operational frequency of modern semiconductor devices. Potential resources of these devices will be exhausted above 40 Gbit/sec which is almost three orders of magnitude less than the frequency required to fully explore the fiber transmission capacity.
WDM method allows, in principle, to utilize the fiber transmission capacity in full. However, to separate adjacent WDM channels, the transmitters and receivers with very stable central frequencies and narrow passbands have to be fabricated. Contemporary active transmitters such as DBR and DFB lasers, and receivers such as fiber gratings, are sophisticated devices vulnerable to environmental fluctuations. Deployment of these devices and feedback circuits required for their stabilization increases complexity and reduces reliability of the existing networks. The projected cost associated with the new generation of WDM equipment having required spectral resolution may be higher than placing additional fiber in the ground.
In a recent attempt to improve the multichannel communications efficiency by better bandwidth utilization a method of Coherence Division Multiplexing (CDM) was suggested based on using phase modulation of partially coherent light. The CDM method employs path-matched white light interferometry which is well known in physics of light sources with a broad optical spectrum. Light Emitting Diodes (LED) and Erbium Doped Fiber Amplifiers (EDFA) are sources having a short coherence length L.sub.C .about.20-30 .mu.k. [see, for example, H. Lefevre, The Fiber-optic Gyroscope, Artech House, Boston, 1993, and references therein]. Two light beams originated from the same LED or EDFA may interfere if the optical paths of these beams differ by less than several coherence lengths. In path-matched interferometry, the path difference L&gt;&gt;L.sub.C between two beams created by a first interferometer is compensated by a second interferometer. Two matched interferometers connected by an optical fiber may be used for signal transmission.
CDM method of multichannel transmission using path-matched interferometry was a subject of a detailed theoretical and experimental analysis. Results of this analysis were described in a number of publications [Wentworth, R. H. et al. "Theoretical noise performance of coherence-multiplexed interferometric sensors", J. of Lightwave Technology 1989, 7, p. 941; Youngquist, R. C. et al. "Selective interferometric sensing by the use of coherence synthesis", Optics Letters, 1987, 12, p. 944; Brooks, J. L. et al. "Coherence multiplexing of fiber-optic interferometric sensors" J. of Lightwave Technology, 1985, LT-3, p. 1062]. Proposals to use CDM for telecommunication were disclosed in U.S. Pat. Nos. 5,549,600, 5,473,459 and 5,606,446.
According to one of the approaches described in the cited articles and patents, the beam of light originated from a short coherence length source 1 is separated into N individual primary channel beams 2 as shown in FIG. 1. On transmitting end 3, transmitting Mach-Zhender interferometers 4 introduce in each of these N primary channel beams 2 a corresponding optical delay L.sub.1, L.sub.2, L.sub.3, . . . L.sub.N wherein each optical delay exceeds several coherence lengths L.sub.C. Each channel beam is phase modulated, and all N beams are combined and transmitted over optical fiber 5. On receiving end 6, the transmitted light is split into N secondary beams, the optical delay is compensated by receiving interferometers 7, and the phase modulation is detected by detectors 8 connected to the output of interferometers 7.
The CDM communication system of FIG. 1 has a serious drawback precluding it from practical implementation. This drawback relates to a low fringe visibility inherent to partially coherent light interferometry. Two interference patterns for partially coherent light for a light source with a uniform spectrum in a spectral range .DELTA..omega. are shown in FIG. 2. Interference pattern 1 is a reference pattern showing intensity as a function of phase .phi. for two interfering beams originated from the same light source, and interference pattern 2 is a channel interference pattern for a CDM system with 5 channels. Each interference pattern has its carrier or central phase equal 0 and -50 rad for interference patterns 1 and 2, respectively), and phase range where fringe visibility is measurable. This phase range corresponds to a difference in optical paths of several coherence lengths, and is approximately 30 rad for the interference patterns 1 and 2 of FIG. 2. Even for the simplest case of two interfering beams, the fringe visibility .gamma. is less than 100%, .gamma.= ##EQU1## where I.sub.max is intensity measured at .phi.=0, and I.sup.min is intensity measured at .phi.-.+-..pi.. For a 5-channel system, when the source output is split into five equal portions before modulation (see FIG. 1), the fringe visibility is less than 40%, as illustrated by interference pattern 2 which is the pattern of one of the working CDM channels. The carrier phase of the this channel is .phi..sub.1 .about.50 rad, or approxiamately 8 central wavelengths. Interference patterns for other channels, not shown in FIG. 2, will have larger phase shifts (for example, 100 rad, 150 rad etc.) and similar fringe visibility. For each of five channels, only 20% of the full light intensity may be used for signal transmission. Maximum signal may be detected when the interference pattern is shifted by .pi.. The signal detected in each channel of a N-channel system (N is a number of channels), does not exceed 2/N.sup.2 fraction of full light intensity.
The signal to noise ratio in CDM systems was thoroughly studied by Prof. H. J. Shaw and his group at Stanford University, Stanford, Calif. The results of these studies were published in a Ph.D. Thesis [R. H. Wentworth, Optical Noise in Interferometric Systems Containing Strongly Unbalanced Paths, Stanford University, 1988] and references cited above. It was shown that the CDM system of FIG. 1 has poorer signal to noise ratio than TDM or WDM system of equal capacity, and hence have no potential advantages compared to TDM or WDM systems.
In each of N channels, the incoherent component, or the fraction (N-1)/N.sup.2 of the full intensity, does not carry signal but fluctuates in time and produces noise. Hence, in a detecting system shown in FIG. 1, a signal to noise ratio of detectors 8 is proportional to 1/N. For large N, this ratio is significantly worse than 1/.sqroot.N normally expected from TDM or WDM systems.
In each receiving channel of the prior art system of FIG. 1, the detected signal is related only to one channel, but the detected noise is related to all other transmission channels. Such detection makes the CDM system impractical for multichannel transmission. The prior art CDM systems are not compatible with the existing single channel transmission equipment which require. the fringe visibility close to 100% and quantum limited signal to noise ratio, and thus have no practical value for multichannel telecommunication.